1. Field of the Invention
This invention relates to a radiation image storage panel employed in a radiation image recording and reproducing method utilizing a stimulable phosphor.
2. Description of Prior Arts
For obtaining a radiation image, there has been conventionally employed a radiography utilizing a combination of a radiographic film having an emulsion layer containing a photosensitive silver salt and a radiographic intensifying screen.
As a method replacing the conventional radiography, a radiation image recording and reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, has been recently paid much attention. In the radiation image recording and reproducing method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is used, and the method involves steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to the object at considerably smaller dose, as compared with the conventional radiography. Accordingly, this method is of great value especially when the method is used for medical diagnosis.
The radiation image storage panel employed in the radiation image recording and reproducing method has a basic structure comprising a support and a phosphor layer provided on one surface of the support. Further, a transparent film is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer comprises a binder and stimulable phosphor particles dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when excited with an electromagnetic wave (stimulating rays) such as visible light or infrared rays after having been exposed to a radiation such as X-rays. Accordingly, the radiation having passed through an object or having radiated from an object is absorbed by the phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced in the panel in the form of a radiation energy-stored image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating (scanning) the panel with stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
The radiation image recording and reproducing method is very useful for obtaining a radiation image as a visible image as described hereinbefore, and it is desired for the radiation image storage panel employed in the method to have a high sensitivity and provide an image of high quality (high sharpness, high graininess, etc.), as well as the radiographic intensifying screen employed in the conventional radiography.
When the radiation image recording and reproducing method is practically carried out, the radiation image storage panel is repeatedly used in a cyclic procedure comprising steps of exposing the panel to a radiation (i.e., recording a radiation image), irradiating the panel with stimulating rays (i.e., reading out the recorded radiation image), and exposing the panel to light for erasure (i.e., erasing the remaining energy from the panel). In the cyclic procedure, the panel is moved from one step to the next step through a transfer system, and after one cycle is finished, the panel is usually piled upon other panels and stored.
In the repeated use of the radiation image storage panel involving the transfer and the pile, various troubles are apt to occur. For instance, both of a surface and a back surface of the panel are damaged by physical contact such as rubbing of a surface (the protective film-side surface) of a panel against a back surface (the support-side surface) of another panel, or rubbing of a surface or back surface of a panel against an edge of another panel, when the panel is piled on the other panels or moved from the pile to the transfer system. The radiation image is generally read out by scanning the transparent protective film-side surface of the panel with stimulating rays, and the physical damage given on the surface of the protective film is liable to cause scattering of the stimulating rays in the read-out operation and the like. As a result, the quality of an image such as uniformity of image tends to be deteriorated.
Accordingly, the radiation image storage panel is desired to be kept from damage on the protective film-side surface thereof at a minimum level in the transferring or piling procedure.
The sensitivity of the radiation image storage panel substantially depends on the amount of stimulated emission given by the stimulable phosphor contained therein, and the emission amount depends upon the emission characteristics of the phosphor itself, as well as the intensity of stimulating rays for causing the phosphor to emit light when the stimulating rays do not have an enough intensity.
There is another problem that stimulating rays are not sufficiently absorbed by the radiation image storage panel, since a portion of the stimulating rays is reflected by the panel surface and does not reach the phosphor layer. Particularly in the case of employing a semiconductor laser which is intended to use practically as a source of stimulating rays, it is required to increase effectiveness thereof to the panel owing to its small output power and to enhance the sensitivity of the panel. The great reflection of the stimulating rays on the panel surface brings about double reflection by the read-out system (e.g., photosensor) and the reflected rays then enter the panel to excite areas other than the area to be irradiated (which is called flare phenomenon). Since the image information recorded on the other parts are read out at the same time, the obtained image information becomes less accurate. Also from this viewpoint, the surface reflection of the panel is desired to be as small as possible.
Further, both of the surface and back surface (protective film-side surface and support-side surface) of the radiation image storage panel, which are usually made of polymer materials, are apt to be electrically charged in the transferring and piling procedure by the above-mentioned physical contacts and rubbing a panel surface against transfer means such as roll and belt. Thus electrically charged panel causes certain problems. For instance, a charged surface of a panel and a charged back surface of another panel easily adhere to each other and thus combined two panels are together moved to the transfer system, so that the subsequent procedure cannot be performed nomarlly. The dust in the air also tends to deposit to the charged panel surface, so that stimulating rays are scattered by the dust on the panel surface in the read-out operation to provide an image of lowered quality. Accordingly, it is also desired to reduce the electrification of the panel surface at a minimum level.